1. Technical Field
The presently disclosed subject matter relates to a high pressure discharge lamp, and in particular, to a high pressure discharge lamp for use in a vehicle lighting device as a light source in place of a halogen lamp, for example. The high pressure discharge lamp made in accordance with principles of the disclosed subject matter can be driven with a decreased starting voltage and can miniaturize the entire lamp device.
2. Description of the Related Art
FIG. 1 shows the configuration of a conventional high pressure discharge lamp 90 of this type. The high pressure discharge lamp 90 is configured to include an arc tube 91 and an outer tube 92 surrounding the arc tube 91. The arc tube 91 has a discharge space 91b containing a rare gas sealed thereinside, and a pair of discharge electrodes 91a projecting into the discharge space 91b and arranged oppositely relative to each other. In the space 95 formed between the arc tube 91 and the outer tube 92, a gas 93 capable of dielectric barrier discharge is sealed.
Examples of such a gas 93 capable of dielectric barrier discharge include Ne, Ar, Kr, Xe, F2, Cl2, Br2, I2, N2, and mixtures thereof.
A description will now be given regarding the start operation (light-up operation) of the conventional high pressure discharge lamp 90. First, a starting pulse is applied between the paired discharge electrodes 91a that project into the discharge space 91b of the arc tube 91. At the same time, an electric field is also applied to the space 95 from current feeding conductors 94a and 94b buried in the respective sealing portions of the arc tube 91 by way of the sealing portions which can be made of a dielectric such as quartz glass.
As pointed out above, the space 95 contains a gas 93 that is apt to produce dielectric barrier discharges. If the starting voltage of dielectric barrier discharge is lower than the discharge starting voltage for the arc tube 91, dielectric barrier discharge starts in the space 95 between the arc tube 91 and the outer tube 92 before the electric discharge starts in the discharge space 91b. The starting voltage of dielectric barrier discharge between the arc tube 91 and the space 95 is typically as low as a few kilovolts (kV).
As the dielectric barrier discharge occurs, light having wavelengths corresponding to the type of gas sealed in the space 95 reaches the paired discharge electrodes 91a. Due to photoelectric effect, electrons are generated to induce an electron avalanche, thereby generating a discharge path inside the discharge space 91b. This reduces the starting voltage to a few kilovolts (kV) which otherwise is approximately 20 kV for starting such a typical high pressure discharge lamp. It should be noted that the reference numeral 96 denotes a lead line for connecting the discharge electrode to a power source, 97 denotes a ceramic pipe for protecting the lead line, 98 denotes a socket, and 99 denotes a light shielding cover by means of a black opaque coating applied onto the surface of the outer tube 92 for serving as a part of a light distribution forming means. (See, for example, Japanese Patent Laid-Open Publication No. 2002-304968 and English Abstract, which are hereby incorporated in their entireties by reference.)
The above-described conventional high pressure discharge lamp 90 has a shape of a so-called double end type, and a lead line 96 is provided for applying a high voltage from the tip end side of the outer tube 92. When a hood (not shown) is attached to the system using this type of high pressure discharge lamp 90 in order to shield direct light from the lamp, the lamp 90 is often separated away from the hood and a vehicle body in order to prevent unnecessary discharge between the lamp and the hood or vehicle body. Furthermore, if a headlight system employs a halogen lamp as a light source and the lamp is desired to be replaced with a discharge lamp in order to improve the brightness of the headlight, the existing halogen lamp cannot be easily replaced with this type of discharge lamp.
In addition to this, there is another problem or characteristic in which a certain large size of this type of lamp system is typically used for attachment. Furthermore, such a conventional high pressure discharge lamp 90 may accidentally require an increased starting voltage due to gas leakage from the space between the arc tube 91 and the outer tube 92. Accordingly, excess or additional time for lighting up to a rated level may be required, or in worst cases, the lamp may not light up. Furthermore, such a discharge lamp has a conduction line located outside the outer tube, and therefore the starting voltage can be reduced without leakage.